1. Field of the invention
This invention relates to a rare earth sintered magnet having experienced minimal shrinkage during sintering and a method for preparing the same.
2. Prior Art
As rare earth magnets of high performance, powder metallurgical Sm--Co system magnets having an energy product of 32 MGOe have been produced on a large commercial scale. Also R-T-B system magnets (wherein T stands for Fe or Fe plus Co) such as Nd-Fe-B magnets were recently developed. For example, a sintered magnet is disclosed in Japanese Patent Application Kokai (JP-A) No. 46008/1984. The R-T-B system magnets use inexpensive raw materials as compared with the Sm--Co system magnets. For the manufacture of R-T-B system sintered magnets, a conventional powder metallurgical process for Sm--Co systems (melting.fwdarw.casting.fwdarw.ingot crushing.fwdarw.fine pulverization.fwdarw.compacting.fwdarw.sintering.fwdarw.magnet) is applicable.
Among the R-T-B system magnets, bonded magnets having a magnet powder bound with a resin binder or metal binder have also been used in practice as well as the sintered magnets. Since the bonded magnets maintain their dimensions upon molding substantially unchanged, their dimensional precision is high enough to eliminate shaping after their manufacture. However, the commercially available R-T-B system bonded magnets are difficult to impart anisotropy by molding in a magnetic field because they use polycrystalline particles containing crystallites prepared by a quenching technique such as a single chill roll technique. Ground powders of R-T-B system sintered magnets cannot be used as a source powder for bonded magnets because they suffer from a drastic decline of coercivity due to strains and oxidation by grinding. It was also proposed to react a ground powder of an R-T-B system alloy ingot with hydrogen to decompose it into a rare earth element hydride, a T boride, and T and to effect dehydration at a predetermined temperature to precipitate crystallites having aligned crystallographic orientation in discrete particles. Although polycrystalline particles obtained by this process can be oriented in a magnetic field and high coercivity is achieved due to crystallites, the process is complex because of the use of hydrogen and has not been used in practice.
In contrast, in the case of R-T-B system sintered magnets, anisotropic magnets are readily obtained because a powder consisting essentially of single crystal particles is compacted in a magnetic field, and higher properties are available because no binder is used. In the sintering process, however, compacts drastically shrink during sintering reaction. It is difficult to maintain the dimensional precision of compacts because shrinkage occurs randomly. The shrinkage varies with a varying degree of orientation of particles in compacts and a varying density. Anisotropic sintered magnets have different shrinkage factors in the direction of easy axis of magnetization and a direction perpendicular thereto. For a compact having a density of 4.3 g/cm.sup.3, for example, the shrinkage factor is about 22% in the direction of easy axis of magnetization and about 15% in the perpendicular direction and the density reaches 7.55 g/cm.sup.3 after sintering.
Such dimensional changes in anisotropic sintered magnets are serious particularly with thin walled, ring or plate-shaped magnets. This is because deflection occurs if a thin walled magnet have uneven shrinkage factors. Then sintered bodies are machined for correcting such dimensional changes before they are marketed. However, the machining process has the problems described below.
(1) Machining of sintered bodies entails a great loss of material. For example, if a deflection of 1 mm occurs in the manufacture of a thin plate-shaped magnet of 1 mm thick, a sintered body of about 3 mm thick must be first produced and then machined at its upper and lower surfaces, resulting in a loss of 2/3 of the material. Such a loss might be avoided by an approach of cutting a plurality of thin plate-shaped magnets out of a single thick block to a thickness of 1 mm, but a loss of about 40% occurs if the machining cutter has a cutting edge width of 0.6 mm. Due to their low mechanical strength, thin wall sintered bodies are liable to chip or crack by impacts during machining or during handling, resulting in a low manufacturing yield.
(2) Magnetic properties become poor. It is precisely reported in the literature that the coercivity of Nd.sub.2 Fe.sub.14 B system sintered magnets depends on the presence of a Nd-rich phase in the grain boundary. In machining sintered magnets of this system, stresses cause cracks to occur along grain boundaries in a region near the machined surface, and coercivity is lost in a region extending from the machined surface to a depth of 0.1 to 0.2 mm. A loss of magnet properties in proximity to the surface being machined is negligible in the case of thick wall magnets, but detrimental in the case of thin wall magnets so that the magnets as a whole show an apparent loss of magnetic properties. It is possible to remove by acid etching the region where coercivity is lost by machining although a material loss of the sintered body is further increased to raise the manufacturing cost.
Under the circumstances, Sm--Co system bonded magnets are generally used for thin wall anisotropic magnets having a longitudinal length/thickness ratio of at least 10, leaving the problem of an increased cost. Thin wall sintered magnets of the R-T-B system are available, but essentially require machining for dimensional adjustment wherein the material yield during machining is 20 to 30%, also raising the problem of an increased cost.